MFde: Update the german documentation set.

Chase some of the latest changes to an article as well as
the {developers-,freebsd-,porters-}handbook.

articles/contributing-ports/article.sgml 1.4 -> 1.5
books/developers-handbook/tools/chapter.sgml 1.50 -> 1.51
books/handbook/txtfiles.ent 1.2 -> 1.4
books/handbook/bibliography/chapter.sgml 1.86 -> 1.87
books/handbook/filesystems/chapter.sgml 1.8 -> 1.9
books/handbook/introduction/chapter.sgml 1.135 -> 1.136
books/handbook/jails/chapter.sgml 1.18 -> 1.20
books/handbook/virtualization/chapter.sgml 1.21 -> 1.22
books/porters-handbook/book.sgml Upstream fixes by jkois@

Approved by: jkois (mentor)
Obtained from: The FreeBSD German Documentation Project